1. Field of the Invention
This invention relates to a method for preventing freezing in the condensing tubes of a power plant""s air-cooled condenser. Employing the present invention, freezing is prevented by regulating the difference in pressure between the exhaust steam header of the condenser and the air ejector system thereby properly regulating temperature in the condenser. More specifically the present invention relates to an air ejector vacuum control valve which prevents condensing tubes from freezing by regulating condensate flow when ambient temperatures are about or below freezing.
2. Background Art
Some power plants create electricity by burning fuel to create intense heat. The heat is used to vaporize liquid water in pipes near the heat source into steam. The steam inside the pipes, which is under great pressure, is directed to pass over the blades of a turbine generator. The steam forces the turbine generator to spin and creates electricity. See FIG. 1. After the steam is exhausted from the turbine, the steam enters a condensing system. Steam exhausted out of the turbine(s) enters a main steam header. The steam header directs the exhausted steam to condensing tubes where within a series of condensing tubes the steam is cooled and allowed to condense back into liquid water. The water is then piped back to the power plant""s heat source where it is vaporized again and the cycle repeats itself. If the steam is not condensed and the turbine continues to exhaust steam, a back pressure on the exhaust side of the turbine builds. While some back pressure is expected, excessive back pressure reduces work output of the turbine, decreasing the efficiency of the turbine. Back pressure is reduced by condensing the steam after it exhausts from the turbine thereby reducing the back pressure in the system and allowing the generator to run more efficiently.
Several different methods for cooling the steam in condensing tubes to reduce the back pressure are known in the art. One method uses cool water running along the outside of the condensing tubes to cool the steam in the tubes. Another method uses air to cool the condensing tubes. Air-cooled condensers rely on wind blowing over and air being forced over the condensing tubes to cool the tubes and thereby speed up the condensation process. While natural winds and ambient temperatures may help to cool the condensing tubes, large industrial fans near the condensing tubes are also typically used to create the additional air flow necessary to cool the condensing tubes. See FIG. 2.
Several methods for preventing freezing in the condensing tubes have been attempted but the known methods are ineffective or impractical. One method suggests increasing the fan speed in order to condense all or most of the steam in the condensing tubes and prevent steam from entering the air ejector system through condensing tubes. This method is ineffective, however, because as the amount of steam and non-condensible gases entering the air ejector system is reduced, a pressure differential develops between the air ejector system and the exhaust steam header. As the fan speed increases and more of the steam in the condensing tube is condensed, the vacuum in the air ejector system gets stronger. Eventually, the vacuum created by the pressure differential is great enough that the steam uptake limit (the amount of steam that normally can enter the condensing tubes) is overcome and large amounts of steam, water vapor and condensate are quickly sucked up through the condensing tubes into the air ejector system. When the air ejector vacuum overcomes the steam uptake limit of a tube, the condensing tube becomes a xe2x80x9csuper conductorxe2x80x9d of steam, carrying steam and condensate into the air ejector system. Once in the air ejector system (AES), the steam, water vapor and condensate come into contact with severely sub-cooled portions of the condensing tubes where the condensates freeze.
Another known method for preventing freezing attempts to use the vacuum created in the air ejector system to draw warm steam from the lower portion of the condensing tubes up into the upper portion of the condensing tubes. It is intended that the steam warm the upper portion of the condensing tube and maintain the upper section at temperatures above freezing. This method runs the risk of creating xe2x80x9csuper conductingxe2x80x9d tubes as explained above and is generally ineffective in keeping the condensing tubes warm enough to prevent freezing.
A third method suggests that by rotating the angle of the angle of the blades of the fans to slow down the air flow across the tubes, the velocity of the steam entering the tubes will decrease and the amount of condensate entering the AES will be eliminated or reduced. Rotating the angle of the fan blades is an inefficient method for controlling air flow.
Another method involves isolating different sections of the condensing tubes in an air ejector system and creating strong vacuums in the isolated sections for a limited period of time in order to pull the warm steam up into the sub-cooled areas. During this time, the isolated sections do not function as steam condensers. This method is time consuming and ineffective.
What is needed is a system, apparatus and method which prevents the damage malfunction, and freezing caused by the conducting of excessive amounts of steam over subcooled portions of the condensing tubes, while allowing for efficient operation of the electrical generator equipment and condensing system. What is also needed is a system, apparatus and method which prevents the damage and malfunction caused by freezing condensate in the AES when ambient temperatures are below freezing, while allowing for efficient operation of the electrical generator equipment and condensing system.
It is an object of the present invention to prevent damage to a condenser and the condenser tubes as a result of condensate freezing in the condensing tubes. Damage from freezing is the result of an inability to appropriately regulate the flow of steam, condensate and gases in the condensing tubes.
It is also an object of this invention to provide a method of deterring freezing in aircooled condenser that maintains a predetermined pressure differential between the pressure in the air ejector system and the pressure in the exhaust steam header.
It is another object of the present invention to provide a method of deterring freezing in the condensing tubes of a condenser that allows the condenser to run as efficiently as possible when ambient temperatures are below freezing.
It is a further object of this invention to provide a method of deterring freezing in an air-cooled condenser that does not require sections of the air-cooled condenser to be shut down or isolated for warming and that allows all sections of the air-cooled condenser to continue operating in subfreezing ambient temperatures.
It is an object of this invention to provide a method of deterring freezing in an aircooled condenser that does not require a change in the configuration of the fan blades in order to prevent the condensing tubes from conducting condensate into the air ejector system.
It is an additional object of this invention to provide a method of deterring freezing in an air-cooled condenser that effectively balances the need for a pressure differential between the air ejector system and the exhaust steam header to draw steam up through the condensing tube and the need to maintain the back pressure in the air ejector system at a level only slightly below that of the back pressure of the exhaust steam header.
It is another object of this invention to provide a method of deterring freezing within the condensing tubes of an air-cooled condenser that is effective and relatively inexpensive to install, operate, and maintain.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.
To achieve the foregoing objects and in accordance with the invention as embodied and broadly described herein a device and method are provided for deterring freezing within the condensing tubes of a condenser. The present invention comprises a means for measuring the pressure differential between the exhaust steam header and the air ejector system and means for regulating the pressure differential. The pressure differential can be measured directly or calculated. Regulating the pressure differential can be accomplished by one of several methods. A mechanical control valve can be disposed within the air ejector system (AES) which effectively isolates the air ejector pump from the AES header thereby selectively controlling the effect the pump has on pressure within the AES. In an alternative embodiment, an exhaust valve can be disposed within the AES header to selectively control the pressure in the AES header. This can increase or decrease the work load on the air ejector pump, effectively reducing pressure in the AES. In still another embodiment, the pump itself can also be used to regulate pressure in the AES. So long as the pressure differential is maintained at a desired level steam, water vapor, and condensate are prevented from being drawn into the AES.
The invention further comprises means for monitoring and maintaining the tube exterior air temperature of the D-section condensing tubes at near ambient air temperature. When tube exterior air temperatures exceed ambient air temperatures, the warmer temperatures indicate that steam is probably being drawn toward the top of the condensing tube and potentially into the AES. The system is then modulated to further cool the condensing tube and condense the steam before the steam enters the AES. The means for monitoring and maintaining the tube exterior air temperature in the condensing tubes facilitates freeze protection by helping to prevent steam from flowing from the condensing tubes into the air ejector system and by helping to maintain an appropriate pressure differential between the exhaust steam header and the AES.
The present invention allows the temperature in the condensing tubes to remain sufficiently cooled, condensing essentially all of the steam in the condensing tubes. This allows the air-cooled condenser to run as efficiently as possible when ambient temperatures are below freezing without risking damage from freezing.